Several parts of a rasterization pipeline, such as hierarchical depth culling, color-, stencil- and depth-compression, rely on storing per-sample coverage masks. A coverage mask indicates the portion of a pixel covered by a primitive (i.e. a polygon) being rendered.
When using Multi-Sample Anti-Aliasing (MSAA) the number of samples grows rapidly, and therefore many algorithms that rely on storing per-sample coverage masks scale poorly with sample rate. In MSAA, more than one sample is used per pixel. Different rates of MSAA involve different numbers of samples.